The present invention relates generally to invasive devices and methods for treatment of the heart, and specifically to devices and methods for electrical stimulation of the heart muscle.
Demand-responsive pacemakers are known in the art. Such devices provide pulses to pace the heart of a patient at a variable rate, dependent on signals received from the body of the patient.
PCT patent application PCT/IL97/00012, published as WO 97/25098, to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes methods for modifying the force of contraction of at least a portion of a heart chamber by applying a non-excitatory electric field to the heart at a delay after electrical activation of the portion. The non-excitatory field is such as does not induce activation potentials in cardiac muscle cells, but rather modifies the cells"" response to subsequent activation. In the context of the present patent application, the use of such a non-excitatory field is referred to as Excitable Tissue Control (ETC). The non-excitatory field may be applied in combination with a pacemaker or defibrillator, which applies an excitatory signal (i.e., pacing or defibrillation pulses) to the heart muscle.
It is an object of some aspects of the present invention to provide improved methods and apparatus for Excitable Tissue Control (ETC) of the heart so as to enhance hemodynamic performance thereof.
In preferred embodiments of the present invention, a cardiac stimulation device comprises one or more ETC electrodes, at least one sensor for sensing physiological activity, and electronic control circuitry, coupled to the ETC electrodes and sensor. The ETC electrodes and, preferably, the sensor are placed at selected sites in the heart of a patient. Alternatively, the sensor may be placed elsewhere inside or on a surface of the patient""s body. The circuitry receives signals from the sensor and analyzes the signals to determine a measure of the physiological activity. Responsive to the measure, the circuitry drives the stimulation electrodes to provide ETC stimulation so as to enhance contractility of the heart muscle. Preferably, the circuitry assesses the measure so as to determine in which of a plurality of predetermined ranges the measure falls, and controls intensity of the ETC stimulation dependent on the range.
In some preferred embodiments of the present invention, the sensor comprises a heart rate sensor, and the circuitry assigns the measured heart rate to one of several heart rate ranges. Preferably, lower and upper heart rate thresholds are assigned, and the circuitry holds off the ETC stimulation when the heart rate is in a range below the lower threshold or above the upper one. When the heart rate is between the lower and upper thresholds, the circuitry preferably applies the ETC stimulation, while adjusting the intensity of the stimulation according to a predetermined function of the measured heart rate. In this manner, safety of the ETC stimulation is improved, and the intensity of the stimulation is adjusted so as to provide enhancement of the contractility, and hence of hemodynamic performance, when and as needed by the patient. Controlling the ETC stimulation by this method also reduces power consumption by the device and thus increases battery lifetime when the device is implanted in the patient""s body.
In preferred embodiments of the present invention, the circuitry times the application of ETC stimulation so that the stimulation is applied at a fixed time, preferably with a predetermined delay, relative to electrical activation of the heart. The electrical activation is typically due to pacing pulses applied to the heart, but may also be due to the normal sinus rhythm, which is preferably detected by the sensor. In some preferred embodiments of the present invention, the circuitry controls the intensity of the ETC stimulation by counting heart beats in sequence and applying the stimulation only at certain of the beats in the sequence. In the context of the present patent application and in the claims, this mode of intensity control is referred to as duty cycle modulation. The inventors have found that ETC has a cumulative effect on heart muscle contractility over a period of many heart beats, and therefore it is believed that duty cycle modulation is a simple and effective means of controlling the intensity of the stimulation.
PCT patent application PCT/IL97/00235, and the corresponding U.S. patent application Ser. No. 09/254,900, which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference, describe a cardiac output controller using ETC stimulation. Control circuitry receives signals from one or more sensors, indicative of the heart""s activity, and responsive thereto, drives the stimulation electrodes to provide the ETC stimulation to the heart. The effect of the controller on cardiac output is regulated by changing the timing of the non-excitatory stimulation pulse relative to the heart""s activity, preferably relative to the heart""s electrical activity of ECG signals received by the sensor (which comprises a sensing electrode). Alternatively or additionally, the controller changes other pulses characteristics, such as the voltage, current, duration, polarity, shape and frequency of the waveform, and delay of the ETC pulse relative to a pacing pulse or to sensing of an activation potential in the heart. The sensors may also include flow rate sensors, pressure sensors, temperature sensors, oxygen sensors, and other types of sensors known in the art, so as to provide additional signals indicative of hemodynamic conditions, such as cardiac output, blood pressure or blood oxygenation.
PCT patent application PCT/IL97/00236, and the corresponding U.S. patent application Ser. No. 09/254,900, which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference, describe a pacemaker that gives cardiac output enhancement. This pacemaker applies both excitatory (pacing) and non-excitatory (ETC) electrical stimulation pulses to the heart. By applying non-excitatory pulses of suitable strength, appropriately timed with respect to the heart""s electrical activation, the contraction of selected segments of the heart muscle can be increased or decreased, thus increasing or decreasing the stroke volume of the heart.
Further aspects of the ETC are described in U.S. patent application Ser. No. 09/101,723, which is similarly assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. The application corresponds to the above-mentioned PCT patent application PCT/IL97/00012.
Israel patent application 125,424, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a cardiac pacemaker that applies an extended pacing signal to the heart, thus enabling simultaneous pacing and ETC stimulation of the heart. The signal typically comprises a pacing pulse or a periodic waveform, preferably made up of a train of pulses, having an overall duration substantially longer than a pulse duration required for pacing the heart. The pacemaker is controlled to selectively apply either the extended pacing signals or ordinary, standard pacing signals, as indicated by the patient""s transient and long-term hemodynamic needs.
Although preferred embodiments of the present invention are described in terms of certain specific types of sensors, typically sensing electrodes, and methods of applying and controlling the intensity of ETC stimulations, such as duty cycle modulation, the scope of the present invention is in no way limited to these modalities. It will be understood that the principles of the present invention may be applied using any other suitable types of sensors, ETC modalities and methods of controlling ETC stimulation intensity, including (but not limited to) those described in the above-mentioned PCT and Israel patent applications.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for modifying contractility of the heart of a patient, including:
receiving signals from a sensor coupled to the body of the patient indicative of physiological activity;
analyzing the signals to derive a measure of the physiological activity;
applying excitable tissue control (ETC) stimulation to the heart so as to enhance contractility of the heart muscle responsive to the measure.
Preferably, applying the stimulation includes applying electrical signals to stimulate the heart and controlling intensity of the signals responsive to the measure, wherein controlling the intensity includes regulating a duty cycle of the signals relative to a beat rate of the heart.
Further preferably, applying the stimulation includes assessing the measure so as to determine in which of a plurality of predetermined ranges the measure falls, and varying the application of the ETC stimulation dependent on the range. Preferably, assessing the measure includes setting upper and lower thresholds with respect to the measure, and varying the application of the stimulation includes holding off the stimulation when the measure is outside a range between the thresholds.
Preferably, assessing the measure includes associating one or more of the ranges with respective types of physical activity undertaken by the patient, and wherein varying the application includes adapting the stimulation to a demand level associated with the physical activity. Most preferably, adapting the stimulation includes increasing the stimulation responsive to an increasing demand level or alternatively, decreasing the stimulation responsive to an increasing demand level.
In a preferred embodiment, analyzing the signals includes deriving a measure of hemodynamic stress.
Preferably, the measure includes a heart rate.
In another preferred embodiment, receiving the signals includes receiving a signal responsive to motion of the patient.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for stimulating cardiac tissue in the body of a patient, including:
at least one sensor, coupled to the body which generates signals indicative of physiological activity;
one or more stimulation electrodes, which are placed in contact with the heart; and
an electrical control unit, which receives and analyzes the signals from the sensor so as to derive a measure of the physiological activity and which applies an excitable tissue control (ETC) signals to the stimulation electrodes so as to enhance contractility of the heart muscle responsive to the measure.
Preferably, the at least one sensor includes an accelerometer.
Alternatively or additionally, the at least one sensor includes a sensing electrode, wherein the sensing electrode preferably includes one of the stimulation electrodes.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for cardiac rehabilitation therapy including:
receiving signals from a sensor coupled to the body of a patient indicative of physiological activity;
analyzing the signals to derive a measure of the physiological activity, the measure having a range of values;
associating the values of the measure with levels of physical activity undertaken by the patient;
applying electrical stimulation to the heart so as to induce muscular exertion thereof responsive to the level, such that over at least a part of the range, the stimulation is adjusted to reduce the muscular exertion of the heart responsive to an increase in the level of activity.
Preferably, applying the stimulation includes inducing exertion of the heart while the patient is at rest and reducing the exertion when the patient is active. Further preferably, includes applying excitable tissue control (ETC) stimulation to the heart so as to enhance contractility of the heart muscle.